The present invention relates to an improvement in polymeric additives for ultrafiltration membranes and processes for purification of water and the like.
The process of reverse osmosis involves the transport of solvent by pressure gradient larger and opposite to osmotic gradient. Ultrafiltration membranes are employed as suitable mediums for reverse osmosis. In the past, it has been an object of ultrafiltration to produce a purified water permeate that is free of organic contaminants, inorganic contaminants, salts, bacteria and microorganisms.
Components which are retained in an ultrafiltration membrane are called collectively concentrate or retentate. Materials which pass through the membrane are called filtrate, ultrafiltrate or permeate.
Membranes are typically selected on how they transport the fluids therethrough. The rate of transport through a membrane is a function of its permeability, generally referred to as flux. Liquid permeation for example, involves the permeation of feed components from the liquid phase on one side of the membrane to the liquid phase on the other side at a controlled rate.
Microporous membranes can be classified into two general types: one type in which the pores are interconnected, i.e., a closed-cell membrane, and the other type in which the pores are essentially interconnected through passages which may extend from one exterior surface or surface region to another, i.e., an open-celled membrane.
Currently available microfiltration membranes are made from essentially hydrophobic polymers such as cellulose nitrate, cellulose acetate, blends of cellulose nitrate and cellulose acetate, cellulose triacetate, polycarbonate, and polysulfone, as well as other polymers. Membranes of these materials, once formed, are usually rendered hydrophilic by the inclusion of surfactant in the casting solution or by a post membrane formation treatment consisting of immersing the material in an aqueous solution of the surfactant. In both cases, although wettability is imparted, the membranes typically contain between 2 and 6% of water extractable (leachable) materials which are potential contaminants of any filtrate.
Another approach to application of a coating to the microporous membrane is to change the surface of the hydrophobic microporous membrane to a hydrophilic one. This is especially true when polyolefinic films, a preferred type of polymeric material often employed in the manufacture of microporous membranes, are employed. Because these films are not "wetted" with water and most aqueous solutions, they could not be used advantageously in various applications. Such proposals have been put forth in the past to overcome these problems, such as exemplified by U.S. Pat. Nos. 3,853,601; 3,231,530; 3,215,486 and Canadian Patent No. 981,991 which utilize a variety of hydrophilic coating agents or impregnants. Such coating agents or impregnants, although effective for a limited period of time tend to be removed in a relatively short period of time by solutions which contact the membrane.
As discussed above, the selectivity of separation membranes is an important factor in the satisfactory operation of membrane separation processes. In addition, membrane properties such as flux and resistance to chemical, biological and physical degradation also affect the efficiency of separation processes.
Of course, there have been many efforts to develop membranes which function efficiently in separation processes. Typical of such efforts include the development of composite-type membranes such as those disclosed in U.S. Pat. Nos. 4,242,159; 4,260,652; 4,277,344 and 4,388,189. These membranes include a microporous support having coated thereon a thin layer of polymeric material. However, previously known composite membranes have not been completely satisfactory since they can or may exhibit a variety of defects which affect physical, chemical and biological degradation resistance and flux, and thus the overall efficiency of the membrane processes for which they are used.
U.S. Pat. No. 4,268,641 (Koenig et al.) discloses thickening agents comprising copolymers of acrylic acid and polyoxyalkylene(meth)acrylates. The copolymers contain about 1 to about 10 mole % of the nonionic (meth)acrylate. There is no disclosure relating to the formation of ultrafiltration membranes.
U.S. Pat. No. 3,341,627 (Wilkinson) discloses a polymeric anti-static agent comprising polymers prepared from alkylphenoxy polyethylene glycol acrylate monomers. There is no disclosure relating to the formation of ultrafiltration membranes.
U.S. Pat. No. 4,280,970 (Kesting) discloses hydrophilic membranes consisting essentially of the graft copolymer of an active hydrogen containing membrane forming polymer, a diisocyanate grafting link, and an active hydrogen containing polyoxyethylene polymer. The membrane forming polymer can be a cellulosic polymer or nylon. The grafting operation can be performed either before or after membrane formation. IGEPAL CO-990 is disclosed (column 3, line 44). The membranes can be employed in ultrafiltration processes. There is no disclosure with reference to a polymeric additive for ultrafiltration membranes.
U.S. Pat. No. 5,266,391 (Donato et al.) discloses a coated microporous membrane comprising a microporous polymeric film support having specified physical attributes and coated on at least one side with a polymer such as polyethylene oxide. In a preferred embodiment, the support is open-celled.
U.S. Pat. No. 4,976,897 (Callahan et al.) discloses a composite membrane having a microporous support coated with a UV curable polymer composition. The UV curable polymer can have additives such as nonylphenoxypolyoxyethylene surfactants (IGEPAL-type) (column 5, line 30).
U.S. Pat. No. 3,717,689 (Tanaka et al.) discloses copolymers prepared from a monoester and a diester. The monoester is a mixed polyoxyalkylene ester of (meth)acrylic acid. There is no disclosure to the use of the copolymer as additives in the preparation of ultrafiltration membranes.
U.S. Pat. No. 5,525,236 (Wilkinson) discloses a microporous ultrafiltration membrane comprising a hydrophobic polymer and a polymer prepared by the addition polymerization of a polyoxyalkylene alkyl phenyl ester of (meth)acrylic acid. The hydrophobic polymer can be a cellulosic polymer, a polyamide, a polycarbonate or a polysulfone. The polyoxyalkylene-containing polymer can be a copolymer prepared with a vinyl comonomer or monomers which lack the polyoxyalkylene moiety. Said comonomers are present in the copolymer in an amount up to about 10 mole % (Col. 6, line 42).